This unusual duo was discovered 6500 light-years away, near the center of our Milky Way galaxy. The pairing is unexpected because this gas giant exoplanet with a mass similar to Jupiter's is orbiting a white dwarf.
A white dwarf is what remains after a sun-like star swells up to a red giant during the star's evolution. Red giants burn through their hydrogen fuel and expand, consuming any planets near their path. After the star loses its atmosphere, all that remains is the collapsed core -- the white dwarf. This remnant, usually about the size of Earth, continues to cool for billions of years.
Finding an intact planet orbiting a white dwarf raises questions about how it survived the star's evolution into a white dwarf.
By observing the system, researchers were able to determine that the planet and star formed around the same time and the planet survived the star's death. The planet is about 2.8 AU from the star. An AU, or astronomical unit, is the distance between Earth and the sun, or 148 million km.
Previously, scientists believed gas giant planets needed to be much further away to survive a sun-like star's death.
The findings of a new study, published Wednesday in the journal Nature, show that planets can survive this incredibly violent phase of stellar evolution and support the theory that more than half of white dwarfs likely have similar planets orbiting them.
"This evidence confirms that planets orbiting at a large enough distance can continue to exist after their star's death," said Joshua Blackman, lead study author and astronomy postdoctoral researcher at the University of Tasmania, in a statement. "Given that this system is an analogue to our own solar system, it suggests that Jupiter and Saturn might survive the Sun's red giant phase, when it runs out of nuclear fuel and self-destructs."
White dwarfs and Earth's future
When our sun becomes a red giant billions of years from now, it will likely engulf Mercury and Venus -- and perhaps Earth.
"Earth's future may not be so rosy because it is much closer to the Sun," said David Bennett, study coauthor and senior research scientist at the University of Maryland and NASA's Goddard Space Flight Centre, in a statement.
"If humankind wanted to move to a moon of Jupiter or Saturn before the Sun fried the Earth during its red supergiant phase, we'd still remain in orbit around the Sun, although we would not be able to rely on heat from the Sun as a white dwarf for very long."
The Jupiter-like planet was previously discovered through a technique called microlensing, which is used to detect cold planets that are distant from their stars. This same technique can be used to find small, faint white dwarfs. Microlensing happens when a star in close proximity to Earth briefly aligns with a more distant star. The closer star's gravity acts like a magnifying lens and increases the light from the more distant star.
The researchers used the W. M. Keck Observatory in Hawaii, as well as its Near-Infrared Camera, to observe the white dwarf and planet. The white dwarf is 60 percent the mass of our sun, and the planet is about 40 percent more massive than Jupiter.
The high-resolution near-infrared images allowed the researchers to rule out the possibility that what the exoplanet is orbiting is a normal star or another type of evolved star.
"We have also been able to rule out the possibility of a neutron star or a black hole host. This means that the planet is orbiting a dead star, a white dwarf," said study coauthor Jean-Philippe Beaulieu, Warren chair of Astrophysics at the University of Tasmania and director of research for the French National Centre for Scientific Research's Institut d'Astrophysique de Paris, in a statement. "It offers a glimpse into what our solar system will look like after the disappearance of the Earth, wiped out in the cataclysmic demise of our Sun."
More surviving planets around white dwarfs
So far, only giant planets have been detected around white dwarfs, but that doesn't mean they are the only planets in existence around these dead stars.
"There should also be smaller mass planets orbiting white dwarfs," Bennett wrote in an email. "Our microlensing surveys detect similar numbers of Jupiters and Neptunes, but we are more sensitive to Jupiters. So, we've found that Neptune mass planets are about 10 times more common than Jupiters in these wider orbits that will survive the end stages of stellar evolution. We expect that we will find planets of a range of masses orbiting white dwarfs."
"We do expect to find planets with a range of masses orbiting white dwarfs, but smaller rocky planets in close orbits are more likely to have been ripped apart during the red giant phase of its host star's evolution," Blackman added.
The researchers will continue the search for exoplanet survivors orbiting dead stars in the future. The Nancy Grace Roman Space Telescope, slated to launch in 2026, "will conduct a much more sensitive microlensing survey that should find many more planets orbiting white dwarfs," Bennett said.
The telescope will directly image giant planets and survey the planets orbiting white dwarfs across our galaxy, providing scientists with a better ratio of how many are destroyed by stellar evolution and how many survive.
In the last few years, other giant planets have been found orbiting dead stars, including one that is slowly being eaten alive by its zombie star, as well as a giant planet that closely orbits a white dwarf.
"The news of another planet found circling a white dwarf is exciting, offering additional proof that planets exist around dead stars after our paper last year reported on the first one ever found," said Lisa Kaltenegger, director of Cornell University's Carl Sagan Institute. Kaltenegger was not involved in the new study.
While this particular planet is unlikely to be potentially habitable for life, there has been a lot of research focused around the idea of searching for life on planets that could orbit white dwarfs.
"If planets can survive their star's demise, could life as well? The James Webb Space Telescope, launching soon, could very well answer the question," Kaltenegger said. "If life could survive even on planets circling stellar corpses, that would make for an amazing future of our cosmos."